Digital television signal broadcasting has several advantages over traditional analog television signal broadcasting. A digital data channel, as with an analog channel, is susceptible to noise, but so long as the received signal overcomes a threshold signal-to-noise level, the transmission is essentially error-free. Channel distortion and noise can be corrected in a digital system by using adaptive equalizers, as well as various error correction and error tolerance methods. Employing coding techniques overcomes channel-related signal impairments and optimizes bandwidth efficiency. Therefore, digital television signals are less susceptible to image distortion and interference. As a result, digital television signals require less bandwidth for comparable image and sound quality.
A typical digital cable television (CATV) system transforms an analog television (TV) signal into a linear pulse-code-modulation (PCM) digital representation of an image. Processing of the transmitted digital signals is done according to a particular application, the processing including frame synchronization and time-base correction, correction for luminance and chrominance error, image manipulation that allows digitally generated effects and graphics to be added to an image, and data compression.
Converting an analog signal to a digital signal is achieved by means of an analog-to-digital (A/D) converter. The A/D typically consists of four components: a band-limiting antialiasing filter, a sample-and-hold circuit that samples the analog signal, a quantizing unit that divides the range of each analog signal sample into a number of distinct levels, and an encoder that places a specified code on the output data lines for each of the quantized levels.
A receiver of the digital signals typically includes a digital-to-analog (D/A) converter having a digital input register in which the bits of a received word are stored, a decoder for converting the data lines into the number of quantized distinct analog levels, a resampling circuit for correcting distortion error introduced by the sample-and-hold, and a band-limiting filter.
Although a digital communication system can be made essentially error-free, the transmission over a physical medium is still susceptible to misalignment, temperature-related drift, and induced noise and impedance variation. The variations due to the physical medium and the system's analog components can be analyzed using various tests. One such test is a sweep test.
A frequency sweep test involves performing measurements over a range of frequency values in order to obtain frequency response information. In order to frequency sweep test a communication system such as a CATV system, a conventional test setup may use a headend test unit connected to the CATV system at its headend and a remote test unit connected to the CATV system at a desired location. In a conventional test, the headend unit sends frequency sweep test signals over the network to a remote test unit. Telemetry signals are utilized to coordinate the operation of the remote test unit with the headend test unit. The headend test unit sequentially injects test signals at each channel frequency and the remote test unit measures signal strength for each respective frequency. The remote test unit determines the frequency response of the CATV system based the results of the sweep test.
A problem with the conventional frequency sweep test is that it disrupts service to the CATV subscribers because the injected test signal interferes with their reception of the corresponding channel. In order to correct this problem, a conventional sweep test system uses a transmission scheme that has a controller which stores a list of channel frequencies to be swept, so that a test signal is generated and transmitted for a particular channel frequency only if a TV signal is not being transmitted on that channel, and the television signals themselves are used as test signals on those channels having a current TV transmission.
Such prior non-invasive sweep systems, however, are designed for use with analog CATV channels that carry television signals having the NTSC format. To this end, the prior sweep systems perform measurements based upon certain standard pulses within the analog television signal. For example, analog sweep tests often rely on vertical synchronization pulses in the performance of measurements because they are predictable in both magnitude and occurrence. Such analog sweep systems are not applicable to digital television signals, which do not include such pulses (e.g., sync pulses). Moreover, such prior art systems often provide limited resolution, typically a single measurement per channel.
A method and apparatus for sweep testing a digital broadband television signal has been defined by U.S. Pat. No. 6,061,393, issued to Tsui, et al. However, the method therein described only computes an estimate of a system response that is then deconvolved to isolate particular components. In addition, that method requires that an impulse be fed into the system and, thus, can be invasive.
Accordingly, there is a need for a sweep measurement method that is non-invasive and can be used on digital communication channels. There is a further need for such a system that has improved resolution over prior art non-invasive systems.